Self-cleaning cooking device

ABSTRACT

A COOKING DEVICE IN WHICH THE WALLS THEREOF HAVE AN OXIDIZING CATALYTIC COATING IN THE FORM OF A BUILT-UP LAYER PREFERABLY APPLIED AS A GLASS-FRIT-CATALYST MIXTURE AND PARTIALLY FUSED TO FORM A POROUS MATRIX LAYER BONDED TO THE BASE ENAMEL LAYER OF THE WALL WITH THE CATALYST DISPERSED THROUGHOUT THE MATRIX.

United States Patent Inventor William C. Moreland, 11 Export, Pa.

Appl. No. 688,712

Filed Dec. 7, 1967 Patented June 28, 1971 Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

SELF-CLEANING COOKING DEVICE 9 Claims, 3 Drawing Figs.

US. Cl 126/19, 117/70, 252/463 Int. Cl. A2lb l/00 Field of Search 1 17/70;

[56] References Cited UNITED STATES PATENTS 3,271,322 9/1966 Stiles l26/l9X 3,460,523 8/1969 Stiles et a1 126/19 Primary Examiner-Edward G. Favors Attorneys- F. H. Henson and E. C. Arenz ABSTRACT: A cooking device in which the walls thereof have an oxidizing catalytic coating in the form of a built-up layer preferably applied as a glass frit-catalyst mixture and partially fused to form a porous matrix layer bonded to the base enamel layer of the wall with the catalyst dispersed throughout the matrix.

SELF-CLEANING COOKING DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention pertains to the art of self-cleaning cooking devices and particularly to those in which the walls are coated with an oxidizing catalyst.

2. Description of the Prior Art Stiles US Pat. No. 3,266,477 discloses a way in which the walls of cooking devices (e.g. ovens) exposed to grease spatters and other productsresulting from cooking food may be made self-cleaning by providing a catalytic oxidizing surface upon the walls so that food residues thereon can be oxidized and removed during heating of the walls to temperatures well below temperatures required to burn off the food residues. A practical arrangement for accomplishing this would provide distinct advantages'over the currently marketed self-cleaning ovens which simply burn off the food at high temperatures in the order to 800 F. and above.

However, it is believed that the teachings of the noted patents do not, for the most part, lend themselves to usage in a practical way in connection with current facilities of oven manufacturers, and the products made in the specific ways taught in the patent would not yield adequate catalytic action. Further, it is my view that while powdered catalysts are available which have the potential of producing adequate catalytic action, the difficulty lies in finding ways of supporting the catalyst to both yield adequate catalytic action and to have adequate adherence and resistance to abrasion.

Thus, my invention deals with improvements in the manner of application of the catalytic oxidizing surface and the resulting product to the end of attaining a surface having good wear characteristics while presenting an adequate amount of catalyst to afford satisfactory cleaning.

SUMMARY OF THE INVENTION In accordance with the broader aspects of my invention, it is contemplated that a porous matrix layer of some depth be built up on the overlie in bonded relation with a base enamel layer, the porous matrix layer being comprised of what I call an aggregate, a binder, and the catalyst. In its currently preferred form, with which I have had my best results thus far, both the aggregate and the binder are glass frit materials with which a powdered catalyst is mixed. This mixture is then applied to a base enamel layer in such a manner that a porous matrix layer is formed by the glass frit particles bonding to each other and to the base enamel layer, with the catalyst dispersed throughout the matrix. The porous matrix so formed is capable of supporting sufficient catalyst to give satisfactory cleaning results, and with the surface of the porous matrix having a relatively high degree of resistance to abrasion.

DRAWING DESCRIPTION FIGS. 1, 2 and 3 are photomicrographs of wall sections magnified 250 times and illustrating the structural character of three different types of catalytic coatings which are obtained in accordance with the invention.

PREFERRED EMBODIMENTS The catalysts which may be used in carrying out the invention include those identified in the noted Stiles patent, for example. They are effective at temperatures as low as 350 F.; and at higher temperatures, such as 400 to 500 F. and above, have more rapid oxidizing potential. As taught, the catalytic materials include the well-known catalysts for oxidation such as the catalytically active compounds of copper, tin, vanadi um, niobium, bismuth, chromium, molybdenum, tungsten, manganese, rhenium, iron, cobalt, nickel, cerium and other rare earths, as well as compounds of the precious metals or the metals themselves such as ruthenium, rhodium, palladium, and platinum. Further, as there taught, the various catalytic metals will be used as their oxides, cerates, manganates, or manganites, chromates or chromites, or their vanadates.

The metal walls or other surfaces of the cooking device are any of the suitable materials of construction such as iron and steel and the various alloys of these, as well as other metals set forth in the noted patent.

The essence of the present invention of course is the provision of a porous matrix layer which, in its currently preferred form, is comprised of glass frit particles partially fused to each other and to a base layer of conventional glass enamel with the catalytic material dispersed throughout the porous layer.

The drawing illustrates three examples of how the general structural characteristics of the exposed porous matrix layer may vary in accordance with the manner in which the coated wall is built up. In each FIG. the metallic wall (such as an oven liner) is identified by the numeral 10. The base layer of conventional porcelain enamel is identified by the numeral 12. The mostly circular, dark spots 14 in the base enamel layer are voids or bubbles in this layer. The porous matrix layer in each FIG. is identified by the numeral 16.

The porous layer 16 of FIGS. 1 and 2 is comprised of the same materials applied in the same way, the difference between the two panels being the presence in FIG. 2 of an overspray strata 18 which appears as a darkened line between the porous layer 16 and the base layer 12. The same overspray line 18 is visible in FIG. 3.

The principal difference between the porous layer 16 of the sections of FIGS. 1 and 2, and the porous layer 16 of the section of FIG. 3, is that the glass frit used in connection with the catalytic material'for the layer of FIG. 3 includes a mixture of somewhat larger frit particles of a second glass along with a larger part of the somewhat smaller frit particles used in the mixture forming the layers of FIGS. 1 and 2.

The specific manner in which the products illustrated in FIGS. 1-3 were formed will be detailed in the following examples.

EXAMPLE I (FIG. 1)

l. A 4 inch X4 inch cold rolled steel panel 10 is provided with a base coat of porcelain enamel 12 applied in accordance with conventional prior art teachings in the enamel coating industry and fired on the panel at a temperature of about l460- -1480 F.

2. A mixture of glass frit particles and an oxidizing catalyst is prepared in a ball mill, along with a minute quantity of additives mainly helpful in the application of the mixture. grams of the O. Hommel Company frit No. 03748 are ground in a ball mill, 0 trace grams, on a 200 mesh screen. The catalyst is DuPont catalyst No. 5937-188 which comprises rare earth carbonates or oxides, and manganese, cobalt, and nickel oxides. In addition to the 100 parts of glass frit and the 50 parts of catalyst, the mixture includes: 1 part of bentonite to serve as a binder during drying of the coating after spray application and before firing; three-eighths part of urea serving to retard tearing on drying; one-sixteenth part of sodium nitrite which keeps the spray-applied-coating from running; and 8 parts black oxide 0. Hommel Company No. PC-17 for coloring.

3. The glass frit and catalyst mixture with sufficient water to permit the spray application thereof is sprayed onto the porcelain base layer surface to a depth of about 1 lto 2 mils thickness or at a rate of 15 to 20 grams of slip per square foot.

4. After the spray applied layer of frit and catalyst is air dried, the entire panel is fired, at l000 1 100 F. the softening temperature range of the 03748 glass) for 5 minutes. The panel produced in this way is useful as the exposed inner surface of a self-cleaning oven wall.

EXAMPLE II (FIG. 2)

The panel of FIG. 2 is produced in the same way as the panel of FIG. 1 except that before the glass frit and catalyst mixture is applied and fired to form the porous matrix layer 16, the overspray layer 18 is formed on the base enamel layer. The overspray, which for purposes of the terms of this application may be considered as a part of the base layer, is a generally conventional step in prior art teachings for some porcelain enamel surfaces for acid resistance or coloring purposes, for example. However, here the overspray is used to obtain a lower melting glass layer for the porous layer to adhere to, and to provide a greater surface area through irregularity which also aids in adherence. The overspray mixture is essentially comprised of an intermediate temperature glass frit mixed with alumina and colloidal silica particles which is spray applied to the surface of the previously spray applied base layer before the base layer is fired. That is, the base layer 12 of conventional porcelain enamel is spray applied and air dried, and then the overspray strata 18 is spray applied and air dried, and both are then fired together.

In the case of the Examples I and II, little difference could be detected in the usefulness of the product for a self-cleaning oven wall purpose leading to the conclusion that the presence of the overspray 18 is likely not warranted. In both cases, the resistance to abrasion was about the same, the quality of the catalytic action was about the same, and the adherence was about the same.

EXAMPLE III (FIG. 3)

l. The base layer 12 of porcelain enamel and the overspray strata 18 are applied to the steel panel 10 as described in connection with the Examples I and II.

2. The catalyst and glass frit mixture used to make the FIG. 3 porous layer comprises 100 parts of the same 0. Hommel No. 03748 frit as in Example I, 100 parts of larger glass frit particles having a higher softening temperature (e.'g. 1450 F.) than the 03748 frit particles, and W parts of the catalyst of Example I, along with corresponding additives proportioned in accordance with the Example I for the purpose of aiding suspension and application of the mixture. The 100 parts of the larger glass frit particles are prepared by grinding the 1450 F. glass frit in a ball mill and testing 100 gm. samples from the mill until only 8 to 9 grams of the particles are left on a 200 mesh screen. The glass in the mill has then been ground to the desired particle size and can then be added in the desired proportion to the rest of the slip.

3. The mixture of glass frit particles including 100 parts of smaller 200 mesh particles and 100 parts of the larger particle glass is spray applied along with the catalyst and additives up to a depth of 2 to 6 mils thickness to the overspray l8 strata and air dried and then fired at 1000" 1 100 F. for minutes.

The resultant panel of FIG. 3 is considered superior to that of Examples I and II. The resistance to abrasion and wearability of the surface appears superior to that of FIGS. 1 and 2 and the catalytic activity also appears to be improved.

EXAMPLE IV (Not Shown) This product was made in the same way as Example III except only half as much of the large particle glass was used. The results were the same as Example III so far as was observable.

The gist of the invention of course is the provision of the porous matrix layer 16. In each of the examples, the aggregate and binder components are the glass frit particles which are softened during firing of this porous layer to a degree that they bond to each other and to the base enamel layer to fonn the matrix with the catalyst dispersed throughout. In FIGS. 1 and 2 the numeral indicates places at which particles of glass frit appear to have been softened and fused together. At other points the particles have softened and bonded to other frit particles forming a finer porous structure. The large fused areas appear to give the structural strength required for maintaining the mechanical integrity of the matrix and the areas of fine structure tend to trap and hold the catalyst. Thus some of the frit particles may be completely softened and fused together, while the other particles may be only softened such that they adhere to other particles on portions of their exterior surface. It will be appreciated that the softening temperature of the particles alone may be different from their softening temperature as affected by additives and the catalyst. Thus it is the cffective softening temperature that is significant in obtaining bonding in the layer.

It is difficult to identify the catalyst particles as such by observation of the photomicrographs. However some of the darker grey areas in the porous matrix layer of the photomicrographs, such as those identified by the numeral 22 are spaces which, but for the presence of potting compound for purposes of making the photomicrograph, would appear as voids with the catalyst particles distributed in the voids and about the surfaces of the glass frit particles defining the voids. It is believed that some of the voids tend to have only highly limited communication with other voids in the layer, and that part of the catalyst in such voids is loose.

In the FIG. 3 structure, the larger glass frit particles incapable of passing through the 200 mesh screen are identified by the numerals 24. The darker grey areas 26 are spaces mostly filled with potting compound and which, as in the cases of the other examples, would be voids in the usable product with the catalyst dispersed on the surfaces of the structure defining the voids.

While in each of the preceding examples glass frits have been noted as serving as both the aggregate and the binder components of the mixture which is used to form the porous layer, other materials may be found to be suitable for use as the aggregate and as the binder components. For example, standard catalytic supports such as noted in the Stiles patent (e.g. alumina or silica) may be used as the aggregate component, with the binder component being low temperature glass frit of the previous examples, or powdered resins or water glass. The samples I have obtained using these latter materials, as distinguished from the materials of my specific examples, have not in my estimation been nearly as satisfactory as those in which the glass frit is the aggregate and binder.

In this connection, it is noted that the term' aggregate as used in this patent application is used in a sense somewhat akin to its use in concrete work, in that the aggregate component of the mixture is the main part and forms the structural framework of the porous layer. In the cases in which I use both a higher temperature, larger particle size frit, and a lower temperature, smaller particle size frit in the mixture, the higher temperature larger size particles are for the most part the aggregate while the lower temperature glass softens more and serves as the binder. When only one type of glass frit is used in the mixture, to the extent that its surface areas soften and bond to frit surface areas and to the base enamel layer, the frit is to that extent the binder, while the remainder of the frit serves as the aggregate component of the mixture.

It is believed to be of interest to note some of the differences between the teachings of the Stiles patent, and the approach in accordance with my invention that I currently consider preferable. Whereas the Stiles patent teaches one alternate approach utilizing carriers for the catalyst which are highly porous, my preferred approach contemplates the use of glass frit particles which are for all practical purposes imporous in their particle form, and accordingly are much stronger. The Stiles patent also teaches the use of supports for the catalyst which have fusing temperature for the most part substantially in excess of the fusing temperatures of the base layer of the enamel, while my best results have been obtained by using glass frit particles which soften and possibly pennits imbedment or at least direct adherence of the catalytic particles to the support. It is of course important in practicing my invention that the frit particles which form the porous layer do not completely fuse together which would cause the surface to glaze over and preclude a porous structure.

It will be appreciated that in accordance with the invention, various modifications may be made in efforts to improve the catalytic activity of the porous matrix layer. As one example thereof additional catalytic activity may be provided by applying an overspray of a straight catalyst mixture or colloid and applying it to the glass frit and catalyst coating after the coating has been spray applied but before firing. While a substan tial portion of such an overspray of straight catalyst is lost through nonadherence, what does adhere increases the coverage.

Another technique is to make the mixture of glass and catalyst and then melt them together at the fusing temperature of the glass frit component of the glass-catalyst mixture. This resultant product may then be refritted with the resulting frit used in place of the pure frit of my specific examples. This same technique may be used with an excess of the catalyst present during the melt prior to refritting.

Additional catalyst may be added after the porous matrix layer with the normal amount of catalyst has been fired and formed, or added to a matrix fonned by glass alone. The application of the catalyst may be done by painting or spraying or otherwise attempting to impregnate the porous matrix layer with either a slurry or colloidal dispersion of finely divided catalytic material. Refiring may or may not be required. Alternately, the catalytic material may be applied in the form of a dissolved salt of one or more of the catalytic materials, which would require additional treatment by heating or otherwise to break the salts down to the desired chemical form for best catalytic action.

Finally, after a panel according to one of the examples has been formed, it may be treated by applying an appropriate etchant to the panel to remove some of the glass within the porous matrix layer in an effort to reexpose any catalyst partially embedded in the softened glass frit particles and also expose occulded voids.

While the invention has preceded with the description of certain specific catalytic materials, it is noted that any suitable oxidizing catalyst may be used which is operative at the preferred temperatures such as 350 --600 F. and does not lose its catalytic character during processing.

Similarly, it is not believed important that the base enamel coat differs substantially from normal porcelain enamel coatings well known in the prior art, except that it is noted that it is the currently preferred method to use a base layer enamel which has a softening temperature sufficiently in excess of the effective softening temperature of a substantial portion of the glass frit particles in the matrix which is to form the porous matrix layer, that the base layer does not soften or begin to soften to any appreciable extent during the firing of the porou matrix layer.

I claim:

1. A cooking device wall exposed to products resulting from heating food, said wall having a coating comprising a vitreous enamel base layer and an overlying porous matrix layer formed of vitreous frit particles partially fused to bond to each other and to said base layer to form said porous matrix layer, and an oxidizing catalyst distributed throughout said matrix layer. '5

2. A cooking device wall according to claim 1 wherein:

said vitreous enamel base layer has a sofiening temperature in excess of the effective softening temperature of said vitreous frit particles in said matrix layer.

3. A cooking device wall according to claim 1 wherein:

said vitreous frit particles include at least two separate groups of particles, one of said groups including particles of a larger size than the particles in the other group for obtaining a relatively coarse porous layer structure.

4. A cooking device wall according to claim 3 wherein:

said one group of particles including said larger particles has a softening temperature higher than the effective softening temperature of said other group, said particles having said lower softening temperature serving as a binder between the particles of said one group.

5. In the method of applying an oxidizing catalyst to a cooking device wall having a base layer of vitreous enamel, the steps of:

mixing a finally divided oxidizing catalytic material with vitreous frit particles to form a mixture thereof;

applying said mixture to the surface of said base layer; and

heating said mixture to a degree only that said frit particles soften sufficiently to bond to each other and to said base layer at contacting locations to form a porous matrix layer attached to said base layer with said catalytic material dispersed within said matrix layer.

6. In a method according to claim 5:

said vitreous frit particles have an effective softening temperature below that at which said base layer softens.

7. In the method according to claim 5:

said vitreous frit particles include one group of particles of one size and another group of particles of a smaller size, the relatively larger particles serving to provide a coarser structure in said matrix layer than said smaller particles alone provide.

8. In the method according to claim 7:

said particles of one size have a softening temperature gneatg than the softening temperature of said smaller sizeparticles.

9. A cooking device wall exposed to products resulting from heating food, said wall having a coating comprising avitreous enamel base layer and an overlying porous matrix layer comprised of an aggregate component defining the porous structural character of the layer, a binder component serving to bond the aggregate component particles to each other at their points of intersection, and to said enamel base layer where said aggregate component particles contact said base layer, and a finely divided oxidizing catalyst dispersed throughout said matrix layer. 

